Algeria's state prosecutor on Thursday ordered an investigation after a mob lynched a man they accused of sparking the country's deadly wildfires.

The North African country has been in the grip of devastating fires since Monday that cost at least 69 lives.

Video footage posted online Wednesday showed a crowd beating to death 38-year-old Jamal Ben Ismail and setting him ablaze.

They alleged he had started the fires.

The grisly murder was staged in Larbaa Nath Irathen, in the Tizi Ouzou district, one of the worst-hit by the fires.

Those responsible "will receive a severe punishment", the prosecutor said, adding that "odious crimes should not remain unpunished".

Amnesty International called on authorities to investigate Ben Ismail's death, which the Algerian League for the Defence of Human Rights branded as "barbaric and atrocious".

"Scenes of the lynching and torching of the suspected arsonist - a young artist who had come to help put out the fires - are shocking," the Algerian group said.

Ben Ismail's father, quoted by local media, called for "calm" as he urged the authorities to "shed light" on his son's death.

Algeria on Thursday began observing a three-day mourning period for those killed in the fires.

Authorities have said they suspect arson after dozens of fires erupted in a short time.

Four suspected "arsonists" have been arrested, but their identities or suspected motives have not been disclosed.